Dummy Gate Cutting Process and Resulting Gate Structures

ABSTRACT

A method includes forming a dummy gate stack, etching the dummy gate stack to form an opening, depositing a first dielectric layer extending into the opening, and depositing a second dielectric layer on the first dielectric layer and extending into the opening. A planarization process is then performed to form a gate isolation region including the first dielectric layer and the second dielectric layer. The dummy gate stack is then removed to form trenches on opposing sides of the gate isolation region. The method further includes performing a first etching process to remove sidewall portions of the first dielectric layer, performing a second etching process to thin the second dielectric layer, and forming replacement gates in the trenches.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the U.S. Provisional ApplicationNo. 62/927,559, filed Oct. 29, 2019, and entitled “Metal Gate FillProcess and Resulting Gate Structures,” which application is herebyincorporated herein by reference.

BACKGROUND

Metal-Oxide-Semiconductor (MOS) devices are basic building elements inintegrated circuits. An existing MOS device typically has a gateelectrode having polysilicon doped with p-type or n-type impurities,using doping operations such as ion implantation or thermal diffusion.The work function of the gate electrode was adjusted to the band-edge ofthe silicon. For an n-type Metal-Oxide-Semiconductor (NMOS) device, thework function may be adjusted to close to the conduction band ofsilicon. For a P-type Metal-Oxide-Semiconductor (PMOS) device, the workfunction may be adjusted to close to the valence band of silicon.Adjusting the work function of the polysilicon gate electrode can beachieved by selecting appropriate impurities.

MOS devices with polysilicon gate electrodes exhibit carrier depletioneffect, which is also referred to as a poly depletion effect. The polydepletion effect occurs when the applied electrical fields sweep awaycarriers from gate regions close to gate dielectrics, forming depletionlayers. In an n-doped polysilicon layer, the depletion layer includesionized non-mobile donor sites, wherein in a p-doped polysilicon layer,the depletion layer includes ionized non-mobile acceptor sites. Thedepletion effect results in an increase in the effective gate dielectricthickness, making it more difficult for an inversion layer to be createdat the surface of the semiconductor.

The poly depletion problem may be solved by forming metal gateelectrodes or metal silicide gate electrodes, wherein the metallic gatesused in NMOS devices and PMOS devices may also have band-edge workfunctions. Since the NMOS devices and PMOS devices have differentrequirements regarding the work functions, dual-gate CMOS devices areused.

In the formation of the metal gate electrodes, a long dummy gate isformed first, which is then etched, so that portions of the long dummygate are separated from each other. A dielectric material may then befilled into the opening left by the etched portion of the long dummygate. The dielectric material is then polished, leaving a portion of thedielectric material between the remaining portions of the dummy gate.The separated portions of the dummy gate are then replaced with metalgates.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-4, 5A, 5B, 6, 7A, 7B, 7C, 8A, 8B-1, 8B-2, 8C, 9A, 9B, 10, 11A,11B, 12A, 12B, and 12C illustrate the cross-sectional views, the topviews, and the perspective views of intermediate stages in the formationof Fin Field-Effect Transistors (FinFETs) and gate isolation regions ona dummy fin in accordance with some embodiments.

FIGS. 13, 14A, 14B, and 15-19 illustrate the cross-sectional views andthe perspective views of intermediate stages in the formation of FinField-Effect Transistors (FinFETs) and a gate isolation region on ashallow trench isolation region in accordance with some embodiments.

FIGS. 20 through 23 illustrate the cross-sectional views in theformation of Gate-All-Around (GAA) Transistors and gate isolationregions in accordance with some embodiments.

FIGS. 24 and 25 illustrate the formation of gate isolation regionshaving multiple layers in accordance with some embodiments.

FIG. 26 illustrates a process flow for forming FinFETs and gateisolation regions in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Gate isolation regions, Fin Field-Effect Transistors (FinFETs), and themethod of forming the same are provided in accordance with variousembodiments. The intermediate stages in the formation of the gateisolation regions are illustrated in accordance with some embodiments.Some variations of some embodiments are discussed. Embodiments discussedherein are to provide examples to enable making or using the subjectmatter of this disclosure, and a person having ordinary skill in the artwill readily understand modifications that can be made while remainingwithin contemplated scopes of different embodiments. Throughout thevarious views and illustrative embodiments, like reference numbers areused to designate like elements. Although method embodiments may bediscussed as being performed in a particular order, other methodembodiments may be performed in any logical order.

In accordance with some embodiments of the present disclosure, theformation of gate isolation regions includes etching a dummy gate toform an opening, filling the opening with a first dielectric layer and asecond layer, and performing a planarization process. The dummy gate isthen removed. A first etching process is preformed to remove the exposedsidewall portions of the first dielectric layer. A second etchingprocess is then performed to thin the second dielectric layer so thatthe resulting gate isolation region has a concave top-view shape.Replacement gates are then formed on opposing sides of the gateisolation regions.

FIGS. 1-4, 5A, 5B, 6, 7A, 7B, 7C, 8A, 8B-1, 8B-2, 8C, 9A, 9B, 10, 11A,11B, 12A, 12B, and 12C illustrate the cross-sectional views ofintermediate stages in the formation of FinFETs and gate isolationregions on a dummy fin. The corresponding processes are also reflectedschematically in the process flow shown in FIG. 26.

FIG. 1 illustrates a perspective view of an initial structure. Theinitial structure includes wafer 10, which further includes substrate20. Substrate 20 may be a semiconductor substrate, which may be asilicon substrate, a silicon germanium substrate, or a substrate formedof other semiconductor materials. Substrate 20 may be doped with ap-type or an n-type impurity. Isolation regions 22 such as ShallowTrench Isolation (STI) regions are formed to extend from a top surfaceof substrate 20 into substrate 20. The respective process is illustratedas process 202 in the process flow 200 as shown in FIG. 26. The portionsof substrate 20 between neighboring STI regions 22 are referred to assemiconductor strips 24. In accordance with some embodiments of thepresent disclosure, semiconductor strips 24 are parts of the originalsubstrate 20, and hence the material of semiconductor strips 24 is thesame as that of substrate 20. In accordance with alternative embodimentsof the present disclosure, semiconductor strips 24 are replacementstrips formed by etching the portions of substrate 20 between STIregions 22 to form recesses, and performing an epitaxy process to regrowanother semiconductor material in the recesses. Accordingly,semiconductor strips 24 are formed of a semiconductor material differentfrom that of substrate 20. In accordance with some embodiments,semiconductor strips 24 are formed of Si, SiP, SiC, SiPC, SiGe, SiGeB,Ge, or a III-V compound semiconductor such as InP, GaAs, AlAs, InAs,InAlAs, InGaAs, or the like.

STI regions 22 may include a liner oxide (not shown), which may be athermal oxide formed through the thermal oxidation of a surface layer ofsubstrate 20. The liner oxide may also be a deposited silicon oxidelayer formed using, for example, Atomic Layer Deposition (ALD),High-Density Plasma Chemical Vapor Deposition (HDPCVD), Chemical VaporDeposition (CVD), or the like. STI regions 22 may also include adielectric material over the liner oxide, wherein the dielectricmaterial may be formed using Flowable Chemical Vapor Deposition (FCVD),spin-on, or the like.

FIG. 2 illustrates the formation of dielectric dummy strip 25, which maybe formed by etching one of the semiconductor strips 24 to form arecess, and then filling the recess with a dielectric material. Therespective process is illustrated as process 204 in the process flow 200as shown in FIG. 26. The dielectric material may include or be a high-kdielectric material such as silicon nitride. Also, the material ofdielectric dummy strip 25 is selected so that it has a high etchingselectivity with relative to the materials of metal gates (such astungsten and titanium nitride) and the materials of STI regions 22 (suchas silicon oxide). In accordance with some embodiments of the presentdisclosure, the material of dummy strip 25 includes a silicon-basedmaterial such as SiN, SiON, SiOCN, SiC, SiOC, SiO₂, or the like. Inaccordance with alternative embodiments of the present disclosure, thematerial of dummy strip 25 includes a metal-based material (oxide ornitride) such as TaN, TaO, HfO, or the like. The bottom surface ofdielectric dummy strip 25 may be higher than, level with, or lower than,the bottom surfaces of STI regions 22.

Referring to FIG. 3, STI regions 22 are recessed. The respective processis illustrated as process 206 in the process flow 200 as shown in FIG.26. The top portions of semiconductor strips 24 and dielectric dummystrip 25 protrude higher than the top surfaces 22A of the remainingportions of STI regions 22 to form protruding semiconductor fins 24′ anddielectric dummy fin 25′, respectively. The etching may be performedusing a dry etching process, wherein HF₃ and NH₃ are used as the etchinggases. In accordance with alternative embodiments of the presentdisclosure, the recessing of STI regions 22 is performed through a wetetching process. The etching chemical may include HF solution, forexample. The height H1 of dielectric dummy fin 25′ may be equal to,greater than, or smaller than, the height H2 of protruding fins 24′. Inaccordance with some embodiments of the present disclosure, the heightH1 of dielectric dummy fin 25 is in the range between about 50 Å andabout 1,500 Å. The width W1 of dielectric dummy fin 25 may be in therange between about 5 Å and about 500 Å.

In above-illustrated embodiments, the fins may be patterned by anysuitable method. For example, the fins may be patterned using one ormore photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers, or mandrels, may then be used to pattern thefins.

Further referring to FIG. 3, dummy gate stacks 30 are formed on the topsurfaces and the sidewalls of (protruding) fins 24′ and 25′. Therespective process is illustrated as process 208 in the process flow 200as shown in FIG. 26. Dummy gate stacks 30 may include dummy gatedielectrics 32 and dummy gate electrodes 34 over dummy gate dielectrics32. Dummy gate electrodes 34 may be formed, for example, usingpolysilicon, and other materials may also be used. Each of dummy gatestacks 30 may also include one (or a plurality of) hard mask layer 36over dummy gate electrode 34. Hard mask layers 36 may be formed ofsilicon nitride, silicon oxide, silicon carbo-nitride, or multi-layersthereof. Dummy gate stacks 30 may cross over a single one or a pluralityof protruding fins 24′ and 25′ and STI regions 22. Dummy gate stacks 30also have lengthwise directions perpendicular to the lengthwisedirections of protruding fins 24′.

Next, gate spacers 38 are formed on the sidewalls of dummy gate stacks30. The respective process is also illustrated as process 208 in theprocess flow 200 as shown in FIG. 26. In accordance with someembodiments of the present disclosure, gate spacers 38 are formed of adielectric material such as silicon nitride, silicon oxide, siliconcarbo-nitride, silicon oxy-nitride, silicon oxy-carbo-nitride, or thelike, and may have a single-layer structure or a multi-layer structureincluding a plurality of dielectric layers.

In accordance with some embodiments of the present disclosure, anetching step is performed to etch the portions of protruding fins 24′that are not covered by dummy gate stack 30 and gate spacers 38,resulting in the structure shown in FIG. 4. The respective process isillustrated as process 210 in the process flow 200 as shown in FIG. 26.The recessing may be anisotropic, and hence the portions of fins 24′directly underlying dummy gate stacks 30 and gate spacers 38 areprotected, and are not etched. The top surfaces of the recessedsemiconductor strips 24 may be lower than the top surfaces 22A of STIregions 22 in accordance with some embodiments. The spaces left by theetched portions of protruding fins 24′ are referred to as recesses 40.In the etching process, dielectric dummy fin 25′ is not etched. Forexample, protruding fins 24′ may be etched using SiCONi (NF₃ and NH₃),Certas (HF and NH₃), or the like.

Next, epitaxy regions (source/drain regions) 42 are formed byselectively growing a semiconductor material from recesses 40, resultingin the structure in FIG. 5A. The respective process is illustrated asprocess 212 in the process flow 200 as shown in FIG. 26. In accordancewith some embodiments, epitaxy regions 42 include silicon germanium,silicon, silicon carbon, or the like. Depending on whether the resultingFinFET is a p-type FinFET or an n-type FinFET, a p-type or an n-typeimpurity may be in-situ doped with the proceeding of the epitaxy. Forexample, when the resulting FinFET is a p-type FinFET, silicon germaniumboron (SiGeB), GeB, or the like may be grown. Conversely, when theresulting FinFET is an n-type FinFET, silicon phosphorous (SiP), siliconcarbon phosphorous (SiCP), or the like, may be grown. In accordance withalternative embodiments of the present disclosure, epitaxy regions 42are formed of a III-V compound semiconductor such as GaAs, InP, GaN,InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinations thereof, ormulti-layers thereof. After epitaxy regions 42 fully fill recesses 40,epitaxy regions 42 start expanding horizontally, and facets may beformed.

FIG. 5B illustrates the formation of cladding source/drain regions 42 inaccordance with alternative embodiments of the present disclosure. Inaccordance with these embodiments, the protruding fins 24′ as shown inFIG. 4 are not recessed, and epitaxy regions 41 are grown on protrudingfins 24′. The material of epitaxy regions 41 may be similar to thematerial of the epitaxy semiconductor material 42 as shown in FIG. 5A,depending on whether the resulting FinFET is a p-type or an n-typeFinFET. Accordingly, source/drains 42 include protruding fins 24′ andthe epitaxy region 41. An implantation may (or may not) be performed toimplant an n-type impurity or a p-type impurity.

FIG. 6 illustrates a perspective view of the structure after theformation of Contact Etch Stop Layer (CESL) 46 and Inter-LayerDielectric (ILD) 48. The respective process is illustrated as process214 in the process flow 200 as shown in FIG. 26. CESL 46 may be formedof silicon nitride, silicon carbo-nitride, or the like. CESL 46 may beformed using a conformal deposition method such as ALD or CVD, forexample. ILD 48 may include a dielectric material formed using, forexample, FCVD, spin-on coating, CVD, or another deposition method. ILD48 may also be formed of or comprise an oxygen-containing dielectricmaterial, which may be silicon-oxide based such as silicon oxide,Phospho-Silicate Glass (PSG), Boro-Silicate Glass (BSG), Boron-DopedPhospho-Silicate Glass (BPSG), or the like. A planarization process suchas Chemical Mechanical Polish (CMP) process or mechanical grindingprocess is performed to level the top surfaces of ILD 48, dummy gatestacks 30, and gate spacers 38 with each other. In accordance with someembodiments of the present disclosure, the planarization process stopson the top of hard mask 36. In accordance with alternative embodiments,hard mask 36 is also removed during the planarization process, and theplanarization process stops on the top surface of dummy gate electrode34. Accordingly, in some of subsequent figures, hard mask 36 isrepresented using a dashed line to indicate it may or may not exist.

Referring to FIG. 7A, a dummy-gate cutting process is performed byetching dummy gate stacks 30 to form openings 50. The respective processis illustrated as process 216 in the process flow 200 as shown in FIG.26. Dummy gate stacks 30 are thus separated into discrete portions. Toperform the dummy-gate cutting process, an etching mask, which mayinclude a photo resist (not shown), may be formed and patterned. FIG. 7Billustrates a cross-sectional view obtained from the referencecross-sectional 7B-7B as shown in FIG. 7A. In the dummy-gate cuttingprocess, dummy gate stacks 30 are etched in anisotropic processes, untildielectric dummy fin 25′ is exposed. As a result, a portion of dummygate stack 30 is removed. The long dummy gate stack 30 is thus cut intotwo discrete portions 30A and 30B that are disconnected from each other.Each discrete portion of dummy gate stack 30 may cross over one, two, ormore protruding fins 24′ in order to form a single-fin FinFET or amulti-fin FinFET. After the etching of dummy gate stack 30, the etchingmask is removed, for example, in an ashing process.

FIG. 7C illustrates a top view of a portion of the structure shown inFIG. 7A. Each of openings 50 is formed between corresponding gate spacerportions 38A and 38B, which are the parallel opposite portions of gatespacer 38. Gate spacer portions 38A and 38B have sidewalls exposed toopening 50. Dielectric dummy fin 25′ is revealed through openings 50.

Next, openings 50 are filled by layers/regions 52-1 and 52-2, which formgate isolation regions 52, as shown in FIG. 8A. The respective processis illustrated as process 218 in the process flow 200 as shown in FIG.26. Layers/regions 52-1 and 52-2 may be formed of dielectric materials,and hence are referred to as dielectric layers/regions hereinafter,while they may also be formed of non-dielectric materials. Dielectriclayers 52-1 and 52-2 are formed of different dielectric materials or thesame materials having different properties such as different densityvalues. Dielectric layers 52-1 and 52-2 may be selected from the samegroup of dielectric materials including, and not limited to, oxide-baseddielectric materials, nitride-based dielectric materials,oxynitride-based dielectric materials, oxycarbide-based dielectricmaterials, carbide-based dielectric materials, etc. For example,dielectric layers 52-1 and 52-2 may be formed of materials selected fromSiN, SiON, SiOCN, SiC, SiOC, SiO₂, or the like. Layers 52-1 and 52-2 mayalso be formed of non-dielectric materials such as SiGe. In accordancewith some embodiments, dielectric layer 52-1 is formed of an oxide suchas silicon oxide, and dielectric layer 52-2 is formed of a nitride suchas silicon nitride. In accordance with alternative embodiments,dielectric layers 52-1 and 52-2 are formed of a same material such assilicon oxide, but have different porosity values and hence differentdensity values. In accordance with some embodiments, dielectric layer52-1 is denser (with a lower porosity) than dielectric layer 52-2. Also,the dielectric layers 52-1 and 52-2 may be formed of a same material,but formed using different process conditions. For example, dielectriclayer 52-1 and dielectric layer 52-2 may be formed using a highertemperature and a lower temperature, respectively. For example, whendielectric layer 52-1 and dielectric layer 52-2 are formed of siliconoxide, the higher temperature may be in the range between about 400° C.and about 600° C., and the lower temperature may be in the range betweenabout 200° C. and about 400° C. Furthermore, the higher temperature maybe higher than the lower temperature by a difference greater than about50° C., and the difference may be in the range between about 50° C. andabout 300° C. When other materials other than silicon oxide are used,the higher and the lower temperature range may be different from that ofsilicon oxide. In accordance with alternative embodiments, as shown inFIG. 24, gate isolation 52 may include more than two layers such asthree, four, five, or the like, which may be up to ten layers.Regardless of whether formed of different materials or the samematerial, dielectric layers 52-1 and 52-2 may be distinguished from eachother, for example, using X-Ray diffraction, Transmission electronmicroscopy (TEM), or the like.

FIGS. 8B-1 and 8B-2 illustrate the processes for forming gate isolationregion 52. In accordance with some embodiments, as shown in FIG. 8B-1,dielectric layer 52-1 is formed using a conformal deposition method, andhence the thickness T2 (FIG. 8B-2) of its vertical portions is close tothe thickness T1 of its horizontal portions (for example, with athickness difference smaller than about 20 percent). In accordance withsome embodiments, dielectric layer 52-1 is formed using Atomic LayerDeposition (ALD), Plasma Enhanced Atomic Layer Deposition (PEALD),Low-Pressure Chemical Vapor Deposition (LPCVD), Chemical VaporDeposition (CVD), Plasma enhanced Chemical Vapor Deposition (PECVD),Physical Vapor Deposition (PVD), or other applicable deposition methods.Each of the lower layers (such as 52-1, or 52-2 if more layers areformed) may have a thickness T1/T2 in the range between about 3 Å andabout 500 Å in accordance with some embodiments. Dielectric layer/region52-2 fills the remaining space of opening 50 (FIG. 7A) unfilled bydielectric layer 52-1. Dielectric layers 52-1 and 52-2 have someportions higher than the top surfaces of dummy gate stacks 30A and 30B.

Referring to FIG. 8B-2, a planarization process is performed to removeexcess portions of dielectric layers 52-1 and 52-2, leaving gateisolation region 52. The portions of dielectric layers 52-1 and 52-2higher than the top surfaces of dummy gate stacks 30A and 30B areremoved. As a result, dummy gate stacks 30A and 30B are exposed, asshown in FIG. 8C. In the meantime, ILD 48 (FIG. 8A) may also be exposedin accordance with some embodiments. The remaining portions ofdielectric layers 52-1 and 52-2 are in combination referred to as gateisolation regions 52 hereinafter, which include the remaining portionsof dielectric layers 52-1 and 52-2.

As shown in FIG. 8C, gate isolation regions 52 separate the respectivedummy gate stacks 30A and 30B from each other. Gate isolation regions 52and dummy gate stacks 30A and 30B in combination form elongated stripsin the top view, and each of the elongated strips is between oppositeportions 38A and 38B of gate spacer 38.

Dummy gate stacks 30A and 30B are then removed through etching, and theresulting structure is shown in FIGS. 9A and 9B. The respective processis illustrated as process 220 in the process flow 200 as shown in FIG.26. In accordance with some embodiments, dummy gate dielectric 32 isremoved. In accordance with alternative embodiments, dummy gatedielectric 32 is not removed during this process, and is exposed afterthe removal of dummy gate electrodes 34. Accordingly, in FIGS. 9B and10, dummy gate dielectric 32 is shown as dashed to indicate that it mayor may not exist in the respective structure. In these embodiments,dummy gate dielectric 32 may be removed when dielectric layer 52-2 isetched in the process shown in FIGS. 11A and 11B, or may be removedafter the process shown in FIGS. 11A and 11B, and before the formationof replacement gates. Openings 54A and 54B are formed in the space leftby the removed dummy gate electrodes 34 (and possibly dummy gatedielectrics 32). As shown in FIG. 9A, each of openings 54A and 54B isdefined by gate isolation region 52 and gate spacers 38, and openings54A and 54B are further separated from each other by gate isolationregion 52. FIG. 9B illustrates a cross-sectional view obtained from thereference cross-section 9B-9B in FIG. 9A. In accordance with someembodiments of the present disclosure, as shown in FIG. 9B, gateisolation region 52 is wider than the underlying dielectric dummy fin25′. In accordance with alternative embodiments, gate isolation region52 may have a same width as, or may be narrower than, dielectric dummyfin 25′.

Referring to FIG. 10, a first etching process 56 is performed to removethe outer sidewall portions of dielectric layer 52-1, so that thesidewalls of dielectric layer 52-2 are exposed. The respective processis illustrated as process 222 in the process flow 200 as shown in FIG.26. The etching process is isotropic, and may be performed through dryetching or wet etching. The etchant is selected according to thematerials of dielectric layers 52-1 and 52-2, so that there is a highetching selectivity ER₅₂₋₁/ER₅₂₋₂, for example, higher than about 4,wherein etching selectivity ER₅₂₋₁/ER₅₂₋₂ is the etching rate ofdielectric layer 52-1 to the etching rate of dielectric layer 52-2.Accordingly, in the first etching process 56, dielectric layer 52-2 isnot etched.

Referring to FIGS. 11A and 11B, a second etching process 58 is performedto thin dielectric layer 52-2, so that the profile of dielectric layer52-2 is modified. The respective process is illustrated as process 224in the process flow 200 as shown in FIG. 26. The etching process isisotropic, and may be performed through dry etching or wet etching. Theetchant is selected according to the materials of dielectric layers 52-1and 52-2, so that there is a relatively high etching selectivityER₅₂₋₂/ER₅₂₋₁ (the etching rate of dielectric layer 52-2 to the etchingrate of dielectric layer 52-1). Accordingly, dielectric layer 52-2 isetched at a higher rate than in the first etching process 56. On theother hand, etching selectivity ER₅₂₋₂/ER₅₂₋₁ may be kept not too high,so that the corners of dielectric layer 52-1 can still be rounded in thesecond etching process 58. In accordance with some embodiments, etchingselectivity ER₅₂₋₂/ER₅₂₋₁ is in the range between about 2 and about 20.In accordance with some embodiments of the present disclosure, the dummygate dielectric 32 (FIG. 10), if not removed in the process shown inFIGS. 9A and 9B, may be removed in the second etching process 58.

In accordance with some embodiments, when one of the etching processes56 and 58 is performed, depending on the materials of dielectric layers52-1 and 52-2, the etching gas may be selected from the group consistingof Cl₂, HBr, CF₄, CHF₃, CH₂F₂, CH₃F, C₄F₆, BCl₃, SF₆, H₂, HF, NH₃, NF₃,and combinations thereof. Also, gases such as N₂, O₂, CO₂, SO₂, CO,SiCl₄, or combinations thereof may be added to improve the etchingselectivity. Inert gases such as Ar, He, Ne, etc., may be added asdilute gases (carrier gas). For example, in an embodiment in whichdielectric layer 52-1 is formed of SiN, and dielectric layer 52-2 isformed of SiO₂, a fluorine-containing gas such as the mixture of CF₄,O₂, and N₂, the mixture of NF₃ and O₂, SF₆, or the mixture of SF₆ andO₂, etc. may be used to etch dielectric layer 52-1, while the mixture ofNF₃ and NH₃, the mixture of HF and NH₃, or the like may be used to thindielectric layer 52-2. In the first etching process 56 and the secondetching process 58, the plasma source power may be in the range betweenabout 10 watts and about 3,000 watts, the plasma bias power may be lowerthan about 3,000 watts. The pressure of the etching gas may be in therange between about 1 mTorr and about 800 mTorr. The flow rate of theetching gas may be in the range between about 1 sccm and about 5,000sccm.

When wet etching is performed in the first etching process 56 and thesecond etching process 58, still depending on the materials of thedielectric layers 52-1 and 52-2, the respective etching solution foretching the corresponding dielectric layers 52-1 and 52-2 may include HFsolution (with fluorine (F₂) dissolved therein), H₂SO₄, HCl, HBr, NH₃,or the like, or combinations thereof. The solvent may include de-ionizedwater, alcohol, acetone, or the like.

In accordance with alternative embodiments, instead of performing twoetching processes using different etching chemicals, a same etchingprocess may be performed to etch both of dielectric layers 52-1 and52-2. The etchant is selected so that the first dielectric layer 52-1has a lower etching rate than dielectric layer 52-2. In the initialstage, the sidewall portions of dielectric layer 52-1 are etched, whiledielectric layer 52-2 is protected by the sidewall portions ofdielectric layer 52-1. After the sidewall portions of dielectric layer52-1 are removed, the sidewalls of dielectric layer 52-2 are exposed,and both of dielectric layers 52-1 and 52-2 are etched. Since dielectriclayer 52-2 has a higher etching rate than dielectric layer 52-1, it isrecessed laterally faster than dielectric layer 52-1, hence forming theprofile as shown in FIG. 11B. It is appreciated that in accordance withthese embodiments, the etching selectivity ER₅₂₋₁/ER₅₂₋₂ (the etchingrate of dielectric layer 52-1 to the etching rate of dielectric layer52-2) is smaller than 1.0, and is selected to be in certain range thatis not too high and not too low. If the etching selectivityER₅₂₋₁/ER₅₂₋₂ is too high, the sidewalls of gate isolation region 52will be convex (opposite to what is shown in FIG. 11B), not concave. Ifthe etching selectivity ER₅₂₋₁/ER₅₂₋₂ is too low, there is a risk thatdielectric layer 52-2 will be etched-through or even fully removed. Inaccordance with some embodiments, the etching selectivity ER₅₂₋₁/ER₅₂₋₂is in the range between about 0.05 and about 1.

Dielectric layers 52-1 and 52-2 may also be formed of a same materialwith different properties. For example, both of dielectric layers 52-1and 52-2 may be formed of silicon oxide, with dielectric layer 52-2being more porous than dielectric layer 52-1. Accordingly, instead ofperforming two etching processes using different etching chemicals, asame etching process may be performed to etch both of dielectric layers52-1 and 52-2. In the beginning of the etching process, the sidewallportions of dielectric layer 52-1 are etched, while dielectric layer52-2 is protected by the sidewall portions of dielectric layer 52-1.After the sidewall portions of dielectric layer 52-1 are removed, thesidewalls of dielectric layer 52-2 are exposed, and both of dielectriclayers 52-1 and 52-2 are etched. Since dielectric layer 52-2 has a lowerdensity than dielectric layer 52-1, dielectric layer 52-2 has a higheretching rate than dielectric layer 52-1. As a result, the resulting gateisolation region 52 also has the profile as shown in FIGS. 11A and 11B.

Through the etching of dielectric layers 52-1 and 52-2 asaforementioned, the profiles as shown in FIGS. 11A and 11B may beformed. As shown in FIG. 11A, the bottom width of dielectric layer 52-2,the bottom width of dielectric layers 52-1, and the top width ofdielectric dummy fin 25′ are denoted as LD1, LD2, and LD3, respectively.In accordance with some embodiments, bottom width LD1 is smaller thanbottom width LD2. Bottom width LD2 may be equal to or smaller than topwidth LD3. The bottom portions of the sidewalls of gate isolation region52 may have a concave shape. Furthermore, the bottom portions of thesidewalls of gate isolation region 52 are curved and smooth. This smoothand concave profile makes the subsequent formation of replacement gateseasy since there is no undercut that is difficult to fill. For example,dashed lines 60 are drawn to illustrate the curved bottom of a gateisolation region formed using conventional method, in which gateisolation region would be formed of a homogenous material. The dashedlines 60 illustrate that sharp undercuts will be formed directly underthe edge portions of the gate isolation region, which undercuts are verydifficult to be filled by replacement gate.

FIG. 11B illustrates a top view of the structure shown in FIG. 11A. Gateisolation regions 52, due to the etching process as aforementioned, haveconcave sidewalls. For example, the middle part of gate isolation region52 may be the narrowest, while the edge portion of gate isolation region52 contacting gate spacers 38 may be the widest. In FIG. 11B, width(lateral dimension) LD4 is greater than width LD5, and width LD5 isgreater than width LD6. In accordance with some embodiments, the widthdifference (LD4−LD5) may be greater than about 5 Å, and the ratio(LD4−LD5)/LD4 may be greater than about 0.05, and may be in the rangebetween about 0.05 and about 1. Also, the width difference (LD5−LD6) maybe greater than about 5 Å, and the ratio (LD5−LD6)/LD5 may be greaterthan about 0.05, and may be in the range between about 0.05 and about 1.

Also, the angle θ formed between the sidewalls of gate isolation region52 and the sidewalls of the corresponding parts of gate spacers 38 isequal to or greater than 90 degrees, and may be in the range between 90degrees and about 160 degrees. This right or obtuse angle also makes iteasy for the filling of replacement gates in the subsequent processes.

FIGS. 12A, 12B, and 12C illustrate a perspective view, a cross-sectionalview, and a top view, respectively, in the formation of replacement gatestacks 66A and 66B. The respective process is illustrated as process 226in the process flow 200 as shown in FIG. 26. FinFETs 68A and 68B arethus formed, with gate stacks 66A and 66B being the replacement gatestacks of FinFETs 68A and 68B, respectively. Replacement gates 66A and66B share common gate spacers 38A and 38B. Furthermore, both ofreplacement gates 66A and 66B abut the gate isolation region 52.

Replacement gate stacks 66A and 66B include gate dielectrics 62 and gateelectrodes 64. Gate dielectrics 62 may include a high-k dielectricmaterial such as hafnium oxide, zirconium oxide, lanthanum oxide, or thelike, and may also include a silicon oxide layer as an interfacial layerbetween the high-k dielectric material and protruding fins 24′. Inaccordance with some embodiments of the present disclosure, gateelectrodes 64 are formed of a metal, a metal alloy, a metal silicide, ametal nitride, or the like, and may have a composite structure includinga plurality of layers formed of TiN, TiAl, Co, Al, and/or the like. Therespective metals and the structure are selected so that the resultingreplacement gate electrodes 64 have appropriate work functions. Forexample, when the resulting FinFET is an n-type FinFET, the workfunction of gate electrode 60 is lower than 4.5 eV, and when theresulting FinFET is a p-type FinFET, the work function of gate electrode60 is higher than 4.5 eV.

FIG. 12B illustrates a cross-sectional view obtained from the referencecross-section 12B-12B in FIG. 12A. As shown in FIG. 12B, gatedielectrics 62 are in contact with both of dielectric layers 52-1 and52-2 of gate isolation region 52. FIG. 12C illustrates a top view of thestructure shown in FIG. 12A. FIG. 12C illustrates angle θ and itscomplementary angle α. Angle α may be equal to or greater than 90degrees, and may in the range between 90 degrees and about 160 degrees.Since the portions of replacement gates stacks 66 contacting gateisolation region 52 have convex shapes, it is easy to fill replacementgate stacks 66 therein without leaving voids.

FIGS. 13, 14A, 14B, and 15-19 illustrate the cross-sectional views andthe perspective views of intermediate stages in the formation of FinFETsand a gate isolation region in accordance with some embodiments. Theseembodiments are similar to the embodiments disclosed in the precedingembodiments, except that gate isolation region 52, instead of landing ondielectric dummy fin 25′, lands on STI region 22. Unless specifiedotherwise, the materials and the formation processes of the componentsin these embodiments (and the embodiments shown in FIGS. 20-25) areessentially the same as the like components, which are denoted by likereference numerals in the preceding embodiments shown in precedingfigures. The details regarding the formation process and the materialsof the components shown in FIGS. 13, 14A, 14B, and 15-19 may thus befound in the discussion of the preceding embodiments.

FIG. 13 illustrates a first semiconductor strip 24 and a secondsemiconductor strip 24, with a continuous STI region 22 extending fromthe first semiconductor strip 24 to the second semiconductor strip 24.Next, the processes as shown in FIGS. 3-6 and 7A are performed. Theprocess shown in FIG. 2 is skipped, and hence no dielectric dummy fin isformed.

FIG. 14A illustrates a structure after the formation of CESL 46 and ILD48. Furthermore, openings 50 are formed to cut dummy gate stacks 30 intoshorter portions 30A and 30B. FIG. 14B illustrates a cross-sectionalview obtained from the reference cross-section 14B-14B in FIG. 14A.Opening 50 extends all the way to STI region 22, so that dummy gatestack 30A is physically and electrically separated from dummy gatestacks 30B. The top-view shapes of the structure shown in FIGS. 14A and14B are essentially the same as what is shown in FIG. 7C, except that nodielectric dummy fins 25′ are formed, and STI region 22 will be exposedto openings 50.

Next, as shown in FIG. 15, gate isolation region 52 is formed in opening50. The formation details and materials may be found referring to thediscussion of FIGS. 8B-1 and 8B-2. Next, the dummy gate stacks 30A and30B are removed to reveal either dummy gate dielectric 32 or protrudingfins 24′, depending on whether dummy gate dielectric 32 is removed ornot at this time. The resulting structure is shown in FIG. 16.

FIG. 17 illustrates the first etching process 56, in which the sidewallportions of dielectric layer 52-1 are removed, and the sidewalls ofdielectric layer 52-2 are exposed to openings 54A and 54B. FIG. 18illustrates the second etching process 58, so that the profile as shownin FIG. 18 is formed. The values of widths LD1, LD2, and LD3 and therelationship (such as the ratios) between widths LD1, LD2, and LD3 maybe similar to what have been discussed referring to FIG. 11A, and arenot repeated herein. The top-view shape of gate isolation region 52 maybe essentially the same as shown in FIG. 11B. FIG. 19 illustrates theformation of replacement gate stacks 66A and 66B. FinFETs 68A and 68Bare thus formed.

The processes for forming gate isolation regions may also be applied tothe formation of other types of transistors other than FinFETs. Forexample, the processes may be applied on the dummy gate cutting forplanar transistors, Gate-All-Around (GAA) transistors, or the like.FIGS. 20 through 23 illustrate the example embodiments in which gateisolation regions are formed for GAA transistors.

Referring to FIG. 20, two stacked layers 114 and 114′ are formed. Eachof stacked layers 114 and 114′ includes channel layers 110 andsacrificial films 112. The total number of channel layers 110 and thetotal number of sacrificial films 112 may be in the range between, andincluding, 1 and about 10. The material of channel layers 110 andsacrificial films 112 are different from each other. In accordance withsome embodiments, the channel layers 110 are formed of or comprise Si,SiGe, or the like. The sacrificial films 112 may be formed of orcomprise SiGe, SiP, SiOCN, SiC, or the like. Stacked layers 114 and 114′overlap the respective semiconductor strips 24. Dummy gate stacks 30,which include dummy gate dielectric 32, dummy gate electrodes 34, andhard masks 36, are formed on the stacked layers 114 and 114′. Opening 50is formed by etching dummy gate stack 30.

In accordance with some embodiments, the perspective view shape and thetop-view shape of the structure shown in FIG. 20 are essentially thesame as what are shown in FIGS. 14A and 7C, except that no dielectricdummy fins 25′ are formed, and protruding fins 24′ are replaced bystacked layers 114 and 114′. The formation processes may be contemplatedreferring to the preceding embodiments.

Referring to FIG. 21, gate isolation region 52 is formed. Dummy gatestacks 30A and 30B are then removed, resulting in trenches 54A and 54B,as shown in FIG. 22. In subsequent processes, the first etching process56 (FIG. 17) and the second etching process 58 (FIG. 18) are performedto modify the profile of gate isolation region 52. The top-view shape ofthe structure shown in FIG. 22 is similar to what is shown in FIG. 11B,except that the protruding fins 24′ in FIG. 11B are replaced by stackedlayers 114 as in FIG. 22.

In subsequent processes, sacrificial films 112 are removed, followed bythe formation of replacement gates 66A and 66B, which includes gatedielectrics 62 encircling channel layers 110, and gate electrodes 64filling the remaining spaces between channel layers 110. GAA transistors68A′ and 68B′ are thus formed.

In accordance with some embodiments of the present disclosure, gateisolation region 52 includes two layers such as layers 52-1 and 52-2. Inaccordance with alternative embodiments, gate isolation region 52 mayinclude more layers such as three, four, five, and up to ten layers. Forexample, FIG. 24 illustrates a top view of gate isolation regions 52,which include layer 52-1, layer 52-n, and the layers 52-2 through52-(n−1) (not shown), with integer n being equal to or greater than 2,and equal to or smaller than 10, for example. The formation processincludes depositing layers 52-1 through 52-(n−1) using conformaldeposition methods, with the materials of the layers 52-1 through layer52-n being different from each other, depositing dielectric layer 52-n,and performing a planarization process. FIG. 25 illustrates a top viewof transistors 68A and 68B after gate isolation regions 52 are formed.The profile is similar to what are discussed referring to FIG. 11B, withthe outer layers of gate isolation region 52 being increasingly widerthan the respective inner layers.

The embodiments of the present disclosure have some advantageousfeatures. By forming multi-layer gate isolation regions, and etching themultiple layers, the profiles of the corner regions of the gateisolation regions are shaped, with no undercut and sharp corners beingformed. The formation of replacement gates is thus easier, and it isless likely to have voids being formed.

In accordance with some embodiments of the present disclosure, a methodincludes forming a dummy gate stack; etching the dummy gate stack toform an opening; depositing a first dielectric layer extending into theopening; depositing a second dielectric layer on the first dielectriclayer and extending into the opening; performing a planarization processto form a gate isolation region comprising the first dielectric layerand the second dielectric layer; removing portions of the dummy gatestack on opposing sides of the gate isolation region to form trenches;performing a first etching process to remove sidewall portions of thefirst dielectric layer; performing a second etching process to thin thesecond dielectric layer; and forming replacement gates in the trenches.In an embodiment, in the first etching process, the first dielectriclayer has a higher etching rate than the second dielectric layer, and inthe second etching process, the first dielectric layer has a loweretching rate than the second dielectric layer. In an embodiment, thefirst etching process and the second etching process results in the gateisolation region to have concave sidewalls facing the trenches. In anembodiment, the method further includes forming a dielectric dummy finprotruding out of isolation regions that are on opposing sides of thedielectric dummy fin, and the gate isolation region has a bottom surfacecontacting the dielectric dummy fin. In an embodiment, the methodfurther includes forming a shallow trench isolation region extendinginto a semiconductor substrate, wherein the gate isolation region has abottom surface contacting the shallow trench isolation region. In anembodiment, the dummy gate stack extends on two neighboringsemiconductor fins. In an embodiment, the dummy gate stack extends ontwo neighboring stacks of stacked layers, and each stack of the stackedlayers comprises alternating channel layers and sacrificial films, andthe method further comprising removing the sacrificial films.

In accordance with some embodiments of the present disclosure, astructure includes a first semiconductor region and a secondsemiconductor region; a first gate stack and a second gate stack on thefirst semiconductor region and the second semiconductor region,respectively; a dielectric region between the first semiconductor regionand the second semiconductor region; and a gate isolation region betweenthe first gate stack and the second gate stack, wherein a bottom surfaceof the gate isolation region contacts the dielectric region, and whereinin a plane view of the gate isolation region, the gate isolation regionhas concave sidewalls in contact with the first gate stack and thesecond gate stack. In an embodiment, the structure further includes afirst gate spacer and a second gate spacer on opposing sides of, andcontacting, the gate isolation region. In an embodiment, each of thefirst gate spacer and the second gate spacer further contacts the firstgate stack and the second gate stack. In an embodiment, the gateisolation region has a bottom part in contact with the dielectricregion, and wherein upper portions of the bottom part are narrower thanrespective lower portions of the bottom part. In an embodiment, the gateisolation region comprises a first dielectric layer and a seconddielectric layer. The first dielectric layer comprises a bottom portion,and two sidewall portions over and connecting to opposing ends of thebottom portion. The second dielectric layer is between the two sidewallportions. In an embodiment, the first dielectric layer and the seconddielectric layer are formed of different materials. In an embodiment,the first dielectric layer and the second dielectric layer are formed ofa same material, and the first dielectric layer and the seconddielectric layer have different porosity values.

In accordance with some embodiments of the present disclosure, astructure includes a first gate stack and a second gate stack. The firstgate stack includes a first gate dielectric; and a first gate electrodeoverlapping a first bottom portion of the first gate dielectric. Thesecond gate stack includes a second gate dielectric; and a second gateelectrode overlapping a second bottom portion of the second gatedielectric. The structure further includes a first gate spacer; and agate isolation region between the first gate stack and the second gatestack, wherein the gate isolation region comprises a first dielectriclayer comprising a bottom portion, and two sidewall portions over andconnected to opposing ends of the bottom portion, wherein the firstdielectric layer forms a first interface with the first gate stack, anda second interface with the first gate spacer, and the first interfaceand the second interface form an acute angle; and a second dielectriclayer between the two sidewall portions. In an embodiment, the structurefurther comprises a second gate spacer, wherein both of the first gatespacer and the second gate spacer are in contact with the gate isolationregion. In an embodiment, the first dielectric layer and the seconddielectric layer are formed of different materials. In an embodiment,the first dielectric layer and the second dielectric layer are formed ofa same material and have different density values. In an embodiment,both of the first dielectric layer and the second dielectric layer arein contact with both of the first gate stack and the second gate stack.In an embodiment, the structure further comprises a dielectric regionunderlying and contacting the gate isolation region, wherein thedielectric region forms a first interface with the gate isolationregion, and the bottom portion of the first dielectric layer forms asecond interface with the second dielectric layer, and the secondinterface is shorter than the first interface.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a dummy gate stack;etching the dummy gate stack to form an opening; depositing a firstdielectric layer extending into the opening; depositing a seconddielectric layer on the first dielectric layer and extending into theopening; performing a planarization process to form a gate isolationregion comprising the first dielectric layer and the second dielectriclayer; removing portions of the dummy gate stack on opposing sides ofthe gate isolation region to form trenches; performing a first etchingprocess to remove sidewall portions of the first dielectric layer;performing a second etching process to thin the second dielectric layer;and forming replacement gates in the trenches.
 2. The method of claim 1,wherein in the first etching process, the first dielectric layer has ahigher etching rate than the second dielectric layer, and in the secondetching process, the first dielectric layer has a lower etching ratethan the second dielectric layer.
 3. The method of claim 1, wherein thefirst etching process and the second etching process results in the gateisolation region to have concave sidewalls facing the trenches.
 4. Themethod of claim 1 further comprising forming a dielectric dummy finprotruding out of isolation regions that are on opposing sides of thedielectric dummy fin, and the gate isolation region has a bottom surfacecontacting the dielectric dummy fin.
 5. The method of claim 1 furthercomprising forming a shallow trench isolation region extending into asemiconductor substrate, wherein the gate isolation region has a bottomsurface contacting the shallow trench isolation region.
 6. The method ofclaim 1, wherein the dummy gate stack extends on two neighboringsemiconductor fins.
 7. The method of claim 1, wherein the dummy gatestack extends on two neighboring stacks of stacked layers, and eachstack of the stacked layers comprises alternating channel layers andsacrificial films, and the method further comprising removing thesacrificial films.
 8. A structure comprising: a first semiconductorregion and a second semiconductor region; a first gate stack and asecond gate stack on the first semiconductor region and the secondsemiconductor region, respectively; a dielectric region between thefirst semiconductor region and the second semiconductor region; and agate isolation region between the first gate stack and the second gatestack, wherein a bottom surface of the gate isolation region contactsthe dielectric region, and wherein in a plane view of the gate isolationregion, the gate isolation region has concave sidewalls in contact withthe first gate stack and the second gate stack.
 9. The structure ofclaim 8 further comprising a first gate spacer and a second gate spaceron opposing sides of, and contacting, the gate isolation region.
 10. Thestructure of claim 9, wherein each of the first gate spacer and thesecond gate spacer further contacts the first gate stack and the secondgate stack.
 11. The structure of claim 8, wherein the gate isolationregion has a bottom part in contact with the dielectric region, andwherein upper portions of the bottom part are narrower than respectivelower portions of the bottom part.
 12. The structure of claim 11,wherein the gate isolation region comprises: a first dielectric layercomprising: a bottom portion; and two sidewall portions over andconnecting to opposing ends of the bottom portion; and a seconddielectric layer between the two sidewall portions.
 13. The structure ofclaim 12, wherein the first dielectric layer and the second dielectriclayer are formed of different materials.
 14. The structure of claim 12,wherein the first dielectric layer and the second dielectric layer areformed of a same material, and the second dielectric layer is moreporous than the first dielectric layer.
 15. A structure comprising: afirst gate stack comprising: a first gate dielectric; and a first gateelectrode overlapping a first bottom portion of the first gatedielectric; a second gate stack comprising: a second gate dielectric;and a second gate electrode overlapping a second bottom portion of thesecond gate dielectric; a first gate spacer; and a gate isolation regionbetween the first gate stack and the second gate stack, wherein the gateisolation region comprises: a first dielectric layer comprising a bottomportion, and two sidewall portions over and connected to opposing endsof the bottom portion, wherein the first dielectric layer forms a firstinterface with the first gate stack, and a second interface with thefirst gate spacer, and the first interface and the second interface forman acute angle; and a second dielectric layer between the two sidewallportions.
 16. The structure of claim 15 further comprising a second gatespacer, wherein both of the first gate spacer and the second gate spacerare in contact with the gate isolation region.
 17. The structure ofclaim 15, wherein the first dielectric layer and the second dielectriclayer are formed of different materials.
 18. The structure of claim 15,wherein the first dielectric layer and the second dielectric layer areformed of a same material and have different density values.
 19. Thestructure of claim 15, wherein both of the first dielectric layer andthe second dielectric layer are in contact with both of the first gatestack and the second gate stack.
 20. The structure of claim 15 furthercomprising a third dielectric layer between the first dielectric layerand the second dielectric layer.